Cardiovascular Journal of Africa: Vol 30 No 2 (March/April 2019)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 30, No 2, March/April 2019

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AFRICA

of auscultatory types of devices. The non-auscultatory types

include automated, ambulatory and Doppler ultrasound devices.

Invasive blood pressure monitoring

The invasive (intra-arterial line) blood pressure monitoring

involves placement of a cannula into an artery (usually radial)

with the distal end of the cannula attached to tubing, which is

then connected to a pressure transducer. Infusion of heparinised

saline through the tubing and cannula prevents a blood clot

in the set-up.

Both numerical measurements and graphical

recording of the blood pressure are displayed on a monitor

real time, and this helps with dynamic

management of patients.

With each heartbeat, the blood pressure waveform rises during

systole and drops during diastole, and the average mean arterial

blood pressure over one cardiac cycle is indicated numerically.

In clinical practice, the intra-arterial device is used in the

management of critically ill patients, many of whom have labile

arterial blood pressures. Such clinically ill pregnant women

include those with septic shock, severe obstetric haemorrhage,

eclampsia and other forms of acute organ failure requiring

resuscitation. Arterial blood gas samples may also be obtained

from the arterial line.

Additionally, the validation of a blood pressure device may be

undertaken using an intra-arterial device as a reference standard.

In fact, the validation of neonatal blood pressure devices may

only be performed using an intra-arterial device as a reference

standard.

Non-invasive blood pressure monitoring, on the other hand,

is used in all categories of patients, particularly in non-critically

ill patients.

Non-invasive auscultatory blood pressure-measuring

device

Measurement of blood pressure with an auscultatory device

involves using a functional stethoscope to listen to the sound

produced when blood flows through a partially occluded

brachial artery, as well as the subsequent changes in the sound

prior to non-occlusion of the vessel. The origin of the sound is

not clearly understood

but is thought to emanate from one or

both of the following: turbulence to blood flow and stretching

of the arterial wall.

In this method of blood pressure measurement (mercury/

aneroid sphygmomanometry), the cuff is inflated on the arm

to occlude the brachial artery and is subsequently deflated. The

radial artery in the wrist is palpated during the cuff inflation,

and the pressure at which the arterial pulsation ceases is noted.

Subsequently, the cuff should be inflated further to increase the

pressure by an additional 20–30 mmHg above the point where

the radial pulse is no longer palpable. A stethoscope is placed

on the antecubital fossa, distal to the cuff, to auscultate the

Korotkoff sound during deflation. The first sound (Korotkoff

phase 1) is heard when the pressure in the cuff begins to allow

blood flow through the artery.

The cuff pressure at which Korotkoff phase 1 sound is heard

represents the systolic blood pressure. Direct palpation of the

arterial pulse while the cuff pressure is inflating imprecisely

indicates the systolic blood pressure. The diastolic blood pressure

is denoted by the disappearance of the sound (Korotkoff phase

V). Should the sound not disappear, the pressure at muffling

of the sound (Korotkoff phase IV) denotes the diastolic blood

pressure.

The traditional auscultatory device has a mercury column

and its use is no longer popular in many clinical settings due

to concerns of mercury toxicity. Irrespective, the European

Commission Scientific Committee on Emerging and Newly

Identified Health Risks (SCENIHR) recommends that mercury

sphygmomanometers may be used for the validation of other

blood pressure-measuring devices.

An aneroid device is a good replacement for the mercury

sphygmomanometers. The aneroid device, however, tends to

lose calibration easily, especially due to mechanical trauma.

Therefore, a mobile aneroid device mounted on a wall or tripod

stand is preferable. Re-calibration of an aneroid device is

required at least once every two years.

For research purposes, a

new aneroid sphygmomanometer with a valid calibration status

is recommended for use.

Of note, all blood pressure-measuring devices including

mercury sphygmomanometers require periodic assessment

and recalibration, although the robustness of the equipment

will influence the frequency of this quality check.

Evidence-

based details of auscultatory and automated blood pressure

measurement techniques are described in Table 1.

Automated blood pressure-measuring device

The automated or oscillatory blood pressure monitor functions

on the principle that flow of blood through an artery partially

constricted with a cuff causes the arterial wall to vibrate. Such

vibration does not occur if there is no arterial wall constriction.

To explain, the inflation of the cuff of the oscillatory blood

pressure monitor causes partial or complete constriction of the

artery. A cuff pressure above the systolic blood pressure causes

complete constriction of the arterial wall. A reduction in the

cuff pressure just below the systolic blood pressure causes partial

constriction of the arterial wall. With each cardiac contraction, a

pulsatile blood flow occurs and causes vibrations in the partially

constricted arterial wall.

The air in the cuff conducts and

transfers the vibration to the transducer in the monitor. The

arterial wall vibrations generate pressure pulses of approximately

3 mmHg in the cuff.

It is the transducer that then generates an

electric signal using the transmitted vibrations.

The oscillatory monitor may be classified as inflationary or

deflationary. An inflationary device detects the oscillation in the

arterial wall when the cuff is inflating.

It prevents the need for

the pneumatic cuff to be over-inflated substantially above the

systolic blood pressure. In the deflationary type, the cuff inflates

to a pre-determined pressure above the systolic blood pressure

before deflating at a rate of approximately 4 mmHg per second

to detect the arterial wall oscillations.

As soon as the pressure

in the cuff is below the diastolic blood pressure, the transmission

of oscillometric pulses ceases because the arterial wall will no

longer be constricted. The pressure pulses are subjected to a

proprietary algorithm of the monitor to generate the blood

pressure reading.

Importantly, the pressure at maximum oscillation corresponds

to the mean arterial pressure, while the changes in the oscillatory

amplitude are used to establish the systolic and diastolic blood

pressures.

Automated devices therefore measure the systolic and